Lauderdale and co-authors (2020, see reference below) are seriously questioning the paradigm establishing that marine phytoplankton growth is limited by iron on a global scale.
Iron availability to marine microbes is facilitated by binding with organic molecules which, in turn, are produced by microbes. The authors hypothesize this forms a reinforcing cycle between biological activity and iron cycling that locally matches the availability of iron and other nutrients, leading to global-scale resource co-limitation between macronutrients and micronutrients, and maximizing biological productivity – adding additional iron to the ocean, even in iron-limited regions does not increase overall productivity. Idealized models support this hypothesis, depending on the specific relationships between microbial sources and sinks of organic chelating molecules. The main outcomes of this work are that:
the marine ecosystem appears to have optimized ligand properties through evolutionary selection, beginning at the Neoproterozoic great oxidation event, when the ocean transitioned from an anoxic conditions with abundant free iron to an oxygenated ocean with only trace concentrations of organic-bound iron.
It also confirms that geoengineering by iron fertilization appears to be ineffective in taking up additional carbon, because global primary production is already at it’s maximum rate.
Lauderdale, J. M., Braakman, R., Forget, G., Dutkiewicz, S., & Follows, M. J. (2020). Microbial feedbacks optimize ocean iron availability. Proceedings of the National Academy of Sciences of the United States of America. DOI: https://doi.org/10.1073/pnas.1917277117